This invention is directed to an electronic timepiece wherein the timekeeping circuitry is formed on an integrated circuit chip, and in particular to an electronic timepiece wherein the timekeeping circuitry is formed of bipolar transistors and C-MOS transistors formed on the same integrated circuit chip substrate.
While small sized electronic timepieces such as wristwatches and the like have taken on various forms, such electronic wristwatches each include an oscillator circuit for producing a high frequency time standard signal, divider circuitry for producing low frequency timekeeping signals in response to the high frequency time standard signal, and a display for displaying time. If the display is a hands display, the divider circuit produces a low frequency timekeeping signal having a period of one second, and such signal effects a stepping of the hands display once each second. Alternatively, if a digital display, usually formed of liquid crystals or light emitting diodes is formed, the divider circuit produces timekeeping signals having distinct periods such as one second, one minute, one hour, one month, which signals are applied to appropriate driving and decoding circuitry to effect driving of the digital display elements, which display elements in turn display actual time.
In order to obtain a highly accurate timepiece, the high frequency time standard signal is produced by an oscillator circuit utilizing a quartz crystal vibrator capable of vibrating at a natural frequency of 32,768 as a time standard. Such oscillator circuits use complementary coupled MOS transistors, hereinafter referred to as "C-MOS" transistors. Additionally, such C-MOS transistors are utilized in the driving circuit for driving the display. The use of such C-MOS transistors in electronic wristwatch oscillator and driving circuits is based on the need for obtaining low voltage operation with a minimum of power consumption.
It is noted that when utilized in electronic wristwatches, C-MOS transistors require substantially no static electric power and consume an amount of power proportional to the operating frequency at which the circuitry operates, such power consumption being moderate in the range of 32 KHz. Accordingly, in view of the minimal power consumed by C-MOS transistors, same are usually suitable for use in electronic timepieces. Nevertheless, in circuits for handling currents or for performing linear operation, such C-MOS transistors are not always desirable.
For example, when MOS transistors are utilized in quartz crystal oscillator circuits, the stability of the quartz crystal oscillator circuit is less than that obtainable by utilizing bipolar transistors in the oscillator circuit in view of the wide variations in the threshold value of the MOS transistors, the generally high threshold value of such MOS transistors, and the large changes in intrinsic transconductance (g.sub.m) caused by changes in the operating point. Such lack of stability when utilizing MOS transistors renders it more difficult to obtain a highly accurate timepiece. Moreover, due to the usually large output impedance of MOS transistors, and the large area that same require to absorb a considerable amount of electric current, it is necessary for a driver circuit formed of same to occupy from one-third to one-half of the entire integrated circuit chip to drive the step motor coil when a mechanical hand display is utilized.
Moreover, when light emitting diodes are utilized to form a digital display, driving currents of several 10 mA are required. In such a case, the driver circuit must be formed of bipolar transistor integrated circuitry requiring two integrated circuit chips, one for the bipolar transistors and the other for the C-MOS transistors.